Progress has been made in developing organic electroluminescence displays (hereinafter referred to as “organic LED displays”) in recent years. Use of organic LED displays, for example, in portable telephones is under study.
FIGS. 33 and 34 show an organic LED display 1, which is fabricated by forming an organic hole transport layer 15 and an organic electron transport layer 16 on opposite sides of an organic luminescent layer 14 to provide an organic layer 13 on a glass substrate 11, and forming anodes 12 and cathodes 17 on opposite sides of the organic layer 13. The organic luminescent layer 14 is caused to luminesce by applying a predetermined voltage across the anode 12 and the cathode 17.
The anodes 12 are made from transparent ITO (indium tin oxide), and the cathodes 17, for example, from an Al—Li alloy. The electrodes of each type are prepared in the form of stripes to intersect those of the other type in the form of a matrix. The anodes 12 are used as data electrodes, and the cathodes 17 as scanning electrodes. With one of horizontally extending scanning electrodes selected, voltage in accordance with input data is applied to data electrodes extending perpendicular to the scanning electrode, whereby the organic layer 13 is caused to luminesce at the intersections of the scanning electrode and the data electrodes to give a display of one line. The scanning electrodes are changed over one after anther in the perpendicular direction to scan the matrix in the perpendicular direction to give a display of one frame.
The methods of driving such organic LED displays include the passive matrix driving method wherein the scanning electrodes and the data electrodes are used for time division driving, and the active matrix driving method wherein each pixel is held luminescent for one vertical scanning period. The organic LED display of the active matrix drive type will be described with reference to FIG. 4. Each pixel 52 is provided with an organic EL element 50 comprising a portion of organic layer, a drive transistor TR2 for controlling the passage of current through the EL element 50, a write transistor TR1 which is brought into conduction in response to the application of scanning voltage SCAN by a scanning electrode and a capacitance element C in which charge is stored by the application of data voltage DATA from a data electrode when the write transistor TR1 is in conduction. The capacitance element C applies an output voltage to the gate of the drive transistor TR2.
First, voltage is applied to the scanning electrodes one after another, and a plurality of first transistors TR1 connected to the same scanning electrode are brought into conduction. Data voltage (input signal) is applied to each data electrode as timed with this scanning. Since the first transistor TR1 is in conduction, the data voltage is stored in the capacitance element C.
The operating state of the second transistor TR2 depends on the amount of charge of data voltage stored in the capacitance element C. For example when the second transistor TR2 conducts, current of a magnitude corresponding to the data voltage is supplied to the EL element 50 via the transistor TR2. Consequently, the EL element 50 luminesces with a brightness in accordance with the data voltage. This luminescent state is maintained over one vertical scanning period.
With the organic LED display of the analog drive type, current of a magnitude corresponding to the data voltage is supplied to the EL element 50 to turn on the EL element 50 with a brightness corresponding to the data voltage as described above. On the other hand, organic LED displays of the digital drive type have been proposed in which a multi-level gradation is produced by supplying to an organic EL element 50 a pulse current having a duty ratio in accordance with the data voltage (e.g., JP-A No. 312173/1998).
With organic LED displays of the digital drive type, one field (or one frame) which is the display cycle of one frame is divided into a plurality of (N) subfields (or subframes) SF, and each subfield SF comprises a scanning period and a luminescence period. The scanning periods included in one field all have the same length, but the luminescence periods have varying lengths each equal to nth power of 2 (n=0, 1, 2, . . . N−1). In the illustrated case (N=4), the four luminescence periods have respective lengths of 8, 4, 2, 1, and on-off control of luminescence period realizes expression of a 16-level gradation.
In subfield driving described, scanning voltage is applied to a write transistor TR1 providing each pixel 53 as shown in FIG. 5, within the scanning period in each subfield SF to write binary data to a capacitance element C, and a drive transistor TR2 supplies current corresponding to the binary data to an organic EL element 50 during the subsequent luminescence period. In subfield driving, the line for supplying current to the drive transistor TR2 constituting each pixel 53 is provided with an on/off switch SW as shown in FIG. 5, whereby the EL elements 50 of the pixels can be made simultaneous with respect to the same luminescence starting time and luminescence termination time in the subfield.
With the organic LED display using the subfield driving method described, all horizontal scanning lines of each of the subfields within one field must be scanned, hence the problem of necessitating high-speed scanning for a multi-level gradation or the problem of producing quasi-contours.
Accordingly, an object of the present invention is to provide a display device of the digital drive type which does not require high-speed scanning for producing a multi-level gradation and which will not permit generation of quasi-contours.